There is a growing demand for optical elements having tunable focal lengths, including adaptive optics systems. For example, adaptive optics technology is used in astronomy in order to overcome significant limitations in the image quality of telescopes, caused by atmospheric turbulence. Also, most optical systems have defects in construction or factors in their environment that prevent them from operating optimally. These defects can be continually measured and automatically corrected by adaptive optics systems, which typically include deformable mirrors whose focal lengths can be changed.
At present, methods and systems that are known in the art for actively changing the focal length of an optical system include at least the following three categories. The first category includes large-scale deformable-optics devices. In these devices, the mirror surface is composed of many pixels, each of which has a curvature or angle of inclination that can be altered through an applied voltage to a bulk piezoelectric actuator. The thickness of the deformable portion of each pixel is relatively large, and thus large forces are required and slow speeds are typical. The second category includes electrostatically actuated devices. These devices consist of large membranes suspended over an array of independently controlled electrodes. Voltages applied to these electrodes generate forces on the portion of the membrane in close proximity to the electrodes, causing deflection of the membranes. The shape of the membrane, and thus its curvature and focal length, are determined by the cumulative affects of the electrostatic forces generated by the applied voltages. A third mechanism that provides adaptive focusing uses a lens that translates mechanically along the optical axis, relative to other lenses in the path, thus changing the focal length. These systems have been demonstrated in both macro- and MEMS (microelectromechanical systems) scales. Canon has demonstrated relatively high speeds (approximately 100 Hz bandwidth) with large-aperture lenses, using patented ultrasonic motors that translate one of the optics components within a compound lens along the optical axis. There are also MEMS-based versions which use thin-film piezoelectric cantilevers or similar structures, to move the optical component along the optical axis relative to a static focusing element.
The prior art methods described above suffer from a number of disadvantages. Regarding the first category of devices described above, the capability of current large-scale deformable-optics devices is limited by the large stiffness in typical macro- or mini-mirrors. This stiffness is caused by the appreciable thickness of the actuator and mirror layers. Much larger changes in focal length could potentially be achieved if this stiffness is reduced.
As for electrostatically actuated devices, such devices must avoid electrostatic pull-in onto a planar electrode, in order to maintain a constant curvature for the focusing element. As a result, gaps may be large and forces small. Small electrostatic forces necessitate the use of relatively compliant membranes for the mirrors, which thus limits their mechanical bandwidth. In addition, the non-linear forces generated by the non-uniform gap typically present in such devices will result in non-uniform bending of the plate. The multi-electrode schemes used to compensate for this distortion add additional complexity, particularly when curvature uniformity is required over a large tuning range.
Among mechanically actuated devices, the macroscale systems are typically slow because of the mass of the components. The MEMS versions have a maximum achievable change in focal length that is generally small (typically less than 10 μm), and thus the application is limited to fine tuning.
While piezo-actuated micro-mirrors have been used in conjunction with macro-scale lenses to achieve focusing devices, high-speed micro-lenses with tunable focal lengths have not yet been implemented. A high-speed deformable focusing element could provide new capabilities in numerous applications, such as optical switching, optical storage disks, and scanning confocal microscopy.
For these reasons, there is a need for a system and method that allows for high speed tuning of focal lengths of optical elements (such as mirrors and lenses) over a wide tuning range, and which do not suffer from the advantages described above. In particular, there is a need for high-speed micro-lenses with tunable focal lengths.